1. Field of the Invention
The present invention relates to a scanning charged-particle beam instrument such as a scanning electron microscope.
2. Description of the Related Art
In scanning electron microscopy, an electron beam emitted by an electron gun is focused onto a specimen by condenser lenses and an objective lens. The electron beam is scanned in two dimensions. As the specimen is irradiated with the electron beam, secondary electrons and other electrons are produced. These produced electrons are detected with a detector. The output signal from the detector is supplied to a cathode-ray tube synchronized to the scanning of the electron beam. Thus, a scanned image of the specimen is obtained.
Where an image of a specimen is observed using such a scanning electron microscope, the specimen stage is mechanically translated in the X- or Y-direction, rotated, or tilted to permit the operator to observe an image of a desired area on the specimen. Movements and rotations of the observed area are not limited to mechanical ones. They may also be accomplished by the images-shifting function for controlling the range in which the electron beam is deflected and by the scan rotation function for electrically rotating the direction of the two-dimensional scan of the electron beam.
Where a specimen is rotated mechanically and an observation is made on this scanning electron microscope, X-motion, Y-motion, and rotation are interlinked such that the center of the viewing screen of the monitor acts as the center of rotation. Where specimens are imaged in a scanning electron microscope, various morphologies of specimens are observed. Sometimes, the operator wants to rotate a specimen image under observation.
Conventionally, where a specimen image on the viewing screen is rotated and observed, horizontal and scanning signals are added to a scanning signal applied to a deflector that scans the electron beam directed to the specimen. In this way, the region scanned by the electron beam is rotated.
In this case, as shown in FIG. 1, the observed image can be rotated while taking the center of the observed image as the center of rotation of the image. In FIG. 1, if the image is rotated through 90xc2x0 while taking the center C of the viewing screen D as the center of rotation, an image 1 indicated by the solid line rotates into a posture as indicated by the broken line 2. FIG. 2 shows one example of an eucentric stage.
In FIG. 2, a Z-stage 3 moves in the direction of height, i.e., in the Z direction. A Y-stage 4 moving in the Y direction, an X-stage 5 moving in the X direction, and a rotating stage 6 rotating about the Z axis are placed on the Z-stage 3. A specimen (not shown) is placed on the rotating stage 6. This Z-stage 3 is equipped with an appropriate mechanism to permit tilting.
In the specimen stage assembly of the construction shown in FIG. 2, as long as the optical axis O, the tilting axis, and the axis of rotation are made coincident, the center position of the observed image is kept on the optical axis if the stage is tilted or rotated. Hence, the specimen image can be prevented from escaping when it is tilted or rotated. That is, where a mechanical rotation is made, if the center of rotation of the stage assembly is coincident with the center of the observed image, the image rotates about the center of the viewing screen, in the same way as where the image is rotated by a scanning signal.
However, with the specimen stage assembly structure shown in FIG. 2, if the observed position of the specimen is moved by translation of the X-Y stage (including the X-stage 5 and the Y-stage 6), the center of rotation also moves. As a result, the center of rotation is no longer coincident with the center of the observed image. This is illustrated in FIGS. 3(a) and 3(b). Where the center of mechanical rotation Cm is spaced from the center of image Cd as shown in FIG. 3(a), if the stage assembly is rotated through 90xc2x0 mechanically, an image 1 lying in the center of the viewing screen as indicated by the solid line moves and appears as an image 2 as indicated by the broken line as shown in FIG. 3(b). Consequently, the range of the observed image shifts greatly.
In view of the foregoing, the present invention has been made.
It is an object of the present invention to provide a scanning charged-particle beam instrument in which an observed image is prevented from escaping due to rotation, even if the center of mechanical rotation is not coincident with the center of the image.
The present invention provides a scanning charged-particle beam instrument having a specimen stage assembly that is a combination of an X-Y stage and a rotating stage. The instrument directs a charged-particle beam to a specimen placed on the specimen stage assembly. The beam is scanned in two dimensions across the specimen. The instrument further includes a detector for detecting signals arising from the specimen when the specimen is illuminated with the beam as described above. In response to this, a scanned image of the specimen is displayed on a viewing screen. Let L be the distance between the center of the observed scanned image and the center of rotation of the rotating stage. This instrument is characterized in that it is further equipped with a control means for moving the X-Y stage according to the distance L and the angular position of the rotating stage such that the image rotates always about the center of the viewing screen when the rotating stage is rotated.
Other objects and features of the invention will appear in the course of the description thereof, which follows.